Audio Systems: radio backround noise, ground loop isolator, oem radio

Question
I have a 2004 Elantra.  Whenever I listen to the radio with the engine running (only with AM) I get terrible backround noise.  If I turn the engine off and listen to the same station, it is crystal clear.  Why is there such bad reception with the engine on?  Thanks for your help!

Answer
Jeff,

 sorry has taken me so long i've been under the weather for coming on a week... this problem is strange for such a late model car.. i'm assuming that your still using the OEM radio unit but either way you can purchase one of 2 things to help eliminate the background noise one is a distortion block it has a fancy technical name but they will know exactly what your wanting .. this usually is under 12 bucks and wires in very simply to your car's harness this should take care of the problem ... you also could have a problem with your ground which might require a ground loop isolator but the blocker should do the trick .. hope this helps you out

thanks

CM